1. Field of the Invention
The present invention relates to a mixer circuit, a receiving circuit, a transmitting circuit, and a method for operating a mixer circuit.
2. Description of the Background Art
From the dissertation entitled, “Monolithische Integration von Frequenzumsetzern bis 45 GHz in Silizium und SiGe,” (monolithic integration of frequency converters to 45 GHz in silicon and SiGe), page 22 ff., by Sabine Hackl, February 2002 at the University of Vienna, various types of the Gilbert cell are known as mixer circuits, such as is shown in FIG. 6 by way of example.
The Gilbert cell shown in FIG. 6 includes a load circuit (R), a mixer core for the LO input, and an HF input stage. Each of the input stages has an emitter-coupled transistor pair. If you consider the DC transfer characteristic of this emitter-coupled transistor pair in terms of large-signal behavior, the function is described by tanh. The input of the mixer core is connected to a local oscillator (LO). In addition, the mixer core is connected to the load circuit, and the emitters of the npn transistors of the mixer core are connected to the HF input stage.
Depending on how the input stages are driven, the Gilbert cell is used as a mixer, modulator, demodulator, multiplier, or phase detector.
If both input voltages present at the inputs HF and LO are in the linear region of the transfer characteristic, the circuit is used as a multiplier. Optimization for high linearity with respect to both inputs, and high output power are desirable in this context.
For mixers and modulators, small signals, which are in the linear region of the characteristic, are present at the HF input, while the LO input takes in switching signals that are limited. The circuit is optimized for minimal noise figure and high linearity with respect to the HF input. If high input voltages, which are in the saturation region of the overall circuit, are applied to both inputs, this circuit is used as a phase detector.
Optimal driving of the LO input stage for mixers and modulators is by means of square wave signals. In practice, however, only sinusoidal signals are generally available, which entail losses. These losses have the effect of longer transition times between two states than is the case for square wave signals, which in turn contributes to increasing the noise figure and reducing the gain. In order to keep these losses low, high LO power is necessary. A loss in gain is caused by sinusoidal voltages at the LO input.
The signal to be mixed is fed to the differential HF input of the Gilbert cell. The “bottom” current switching stage serves as an amplifier. In the “top” stage, the signal is switched back and forth between the two load resistors at the rate of the LO signal. (A variant with bipolar transistors as “current switches” is shown here.)
Hence, the signal to be mixed is multiplied by a square wave function. Since a square wave function can also be represented as the sum of sinusoidal oscillations at the fundamental frequency and the odd multiples thereof, this method corresponds to multiplicative mixing with the distinction that a filtering of the output signal should be performed in this case because of the harmonics of the square wave function.
An improved mixer circuit is known, for example, from U.S. Pat. Nos. 6,542,019, 6,348,830 B1 using bipolar transistors, or U.S. Pat. No. 6,480,046 B1 using field-effect transistors. In U.S. Pat. No. 4,590,433, the transistors of the two differential amplifiers are controlled for low power consumption such that either all four transistors of the two differential amplifiers are conductive, or that only one respective transistor of a differential amplifier is non-conductive.